In order to reduce network overhead, the Employee service should also provide the following overload:

Employee [] getEmployeeInfo(int[] employeeId)

Now, should Employee be a struct or a class (a value type or a referenece type)?
Since the array is a reference type, boxing will kick in to convert the value type to a reference type at runtime and performance overhead will be incurred. This makes it sensible to make Employee a class rather than a struct.

Although this explanation cites an SOA-based implementation, it is applicable in any scenario involving boxing.